Electric control units which are intended, for instance, for the control of an internal combustion engine, produce an indication upon the occurrence of defects. For this purpose, the indicating means are actuated over a control line. In connection with the monitoring of safety-relevant devices in motor vehicles, for instance anti-lock brake systems, it is necessary that the control line also be monitored so that it is possible to recognize a break in the control line as well as a short circuit between the control line and the supply voltage. Such a device is described in German Utility Model Application No. G 93 131 291, which has not yet been published. That device is, however, of relatively expensive construction.